The present invention relates to a communication apparatus for converting the frequency of a received signal from a plurality of hand sets and retransmitting the signal. In particular, the present invention relates to a communication apparatus as radio relay means for conversation between hand sets such as a cordless phone, mobile phone and the like.
Conventional communication apparatuses include a cordless phone for conversation between hand sets via a base station wherein adjacent channels are used for conversation between hand sets to perform processing of a radio frequency part and processing of intermediate frequency of radio signals on the same circuit and the frequency of a received radio frequency signal is converted and the signal is retransmitted (Japanese Patent Laid-Open Publication No. Hei 4-180415).
There is also another communication apparatus wherein conversation between hand sets is performed by simplex operation for alternating between transmission and reception depending on the time (Japanese Parent Laid-Open Publication No. Hei 4-342346). This communication apparatus switches the conversation direction by generating/receiving (control) signals for switching between transmission and reception.
Furthermore, there is another communication apparatus which has a radio configuration capable of simultaneous communication on a plurality of channels by a radio with a press-to-talk (simplex) system using a wideband radio transceiving unit (Japanese Patent Laid-Open Publication No. Hei 11-196019).
By the way, the above-described communication apparatuses are advantageous in that conversation between hand sets is enabled by using only one radio unit, but has problems (1) to (4) as below.    (1) In the “communication apparatus for converting the frequency of a received radio frequency signal and retransmitting the signal” of Japanese Patent Laid-Open Publication No. Hei 4-180415, conversation between hand sets is impossible in a communication system with fixed duplex intervals since a transmit signal is obtained within 380 MHz band by synthesizing a 130 MHz signal to a received signal within 250 MHz band.
A basic communication apparatus having a plurality of radio units as shown in FIG. 9, in order to enable conversation between hand sets, a base station is usually provided with a duplexer 151 to which an antenna 152 is connected, two sets of receivers (123–128), a signal processor 140 for processing a signal from each receiver, two sets of transmitters (132–134) to which a signal from the signal processor 140 is connected, two sets of local oscillators 150, a handset 160, telephone line I/F 170 and a control and I/O unit 180. If one hand set transmits a signal at 254.1 MHz when the duplex intervals are fixed at 130 MHz, the base station receives the signal at 384.1 MHz and if the other hand set transmits a signal at 255.1 MHz, the base station receives the signal at 385.1 MHz. Therefore, so that two hand sets communicate via the base station, the base station needs to convert the transmission frequency of one hand set, 254.1 MHz, to the reception frequency of the other hand set, 385.1 MHz, and the transmission frequency of the other hand set, 255.1 MHz, to the reception frequency of this hand set, 384.1 MHz. as shown in FIG. 10, however, there is a problem with the method of the abovementioned publication that since 130 MHz is simply added to a signal received by the base station, a communication system with fixed duplex intervals cannot be employed for this method (254.1 MHz becomes 384.1 MHz and the signal is returned to the first hand set itself).    (2) In the “communication apparatus wherein adjacent channels are used for conversation between hand sets to perform processing of radio frequency part of radio signals and processing of intermediate frequency on the same circuit” of Japanese Patent Laid-Open Publication No. Hei 4-180415, there is a problem that an adjacent unassigned channel needs to be prepared at the start of the conversation and the frequency cannot be arbitrarily selected since adjacent channels are used. Also, since a modulation system of demodulating each channel is employed in this communication apparatus, a circuit for separating each signal at a certain stage during signal processing to handle signals is needed. Therefore, there is a problem that the configuration is complicated and thereby the cost is high.    (3) In the “communication apparatus for achieving conversation between hand sets by a simplex operation system switching between transmission and reception depending on the time” of Japanese Patent Laid-Open Publication No. Hei 4-342346, there is a problem that since simultaneous conversation is impossible, the user accustomed to a duplex operation system finds the simplex operation unnatural. For example, even if one wants to say something while the other person is talking and utter something in the state, nothing is received at the other person.    (4) In the “communication apparatus having a radio configuration capable of simultaneous communication on a plurality of channels by a press-to-talk system using a wideband radio transceiver”, there is also a problem that duplex conversation between hand sets is impossible as is the case with the above (3).